Evaluation of the resonance of an object to various acoustic input frequencies has been known as a method for determining the condition of metallic or otherwise highly rigid objects or components for centuries. In fact, some uses of this principle may pre-date the industrial revolution. With respect to rail, early British railroad engineers would tap the wheels of a train and use the sound to determine if a crack was present. Of course, bell makers rely on resonances for their products to work at all. In one approach, a simple physical striker (similar to a ball-peen hammer under automatic control) and comparison of the resulting acoustic spectra of wheels on both sides of an axle is used to determine if one wheel is bad, under the assumptions that (A) the spectra of two good wheels should be very similar, and (B) that it is very unlikely for two wheels on one axle to have defects so similar as to produce nearly-identical spectra. Note that this second assumption may not be entirely valid. For example, slid-flats are caused by the locking of a brake and the subsequent friction between wheel and rail causing flat wear in one location on the wheel. In this case, one would expect the flat area of wheels on the same axle to be very nearly identical. In another proposal, a preliminary acoustic-signature inspection system using a similar hammer-based approach was designed and tested, but only achieved 45% identification of known flawed wheels with a false alarm rate of almost one third.
Other methods of using sound to determine the presence of a defect in such objects are known. For example, the inventors previously proposed electromagnetically inducing acoustic signals into a wheel (electromagnetic acoustic transduction) and tracking the nature and timing of the returned signals. However, this method, and related ultrasonic inspection methods, are specifically focused on locating and identifying very specific flaws.
Resonant frequencies are dependent on the material characteristics of an object. In general, this relationship may be described as follows:
            f      r        ~                  k        m              ,where fr is a resonant frequency of the object, k is a measurement of the “stiffness” of the object (Young's Modulus), and m is a general symbol for mass which may take into account the dimensions and density of the object. As a complex object is made up of material which may have multiple boundaries and even differing compositions and stresses there within, it is in effect made up of many different sub-objects, and just as a combination of a violin body and string resonate in a specific way, the sub-objects themselves as well as the various combinations of these sub-objects may have resonances. Thus, there may be many thousands of resonances in any given object. In theory, as all components of a heterogeneous solid have their own resonances, and the interaction of these components will introduce resonances and resonance shifts directly related to the size, shape, and composition of those components, it is possible to completely describe the entire object—crystalline structure, inclusions, shape, size, material composition—in terms of its resonances.
While this ultimate application of resonances may be forever relegated to theory due to physical and computational constraints, the important point is that any significant wear on a component will change its dimensions (and thus m and resonant frequencies), and a defect (such as a crack) in a component will change the stiffness of the component in that location, leading to an overall change in k and thus also in one or more of the relevant resonant frequencies. Conversely, for objects manufactured to adequate tolerances which are in good condition, all resonances would be expected to be very close together. This means that a “resonance spectrum”—a scan across all the emission frequencies of the vibrating object which shows all of the significant resonant peaks—for any “good” component should be very similar, and any flawed component will noticeably depart from that spectrum, regardless of what the exact nature of the defect may be. This differs significantly from the previously described approach comparing wheels on opposing sides of an axle as a specific spectrum or spectra are known for “good” components and there is no reliance on assumptions of goodness. Moreover, the previously described approach of comparing wheels on opposing sides of an axle does not focus on resonances, which are specific characteristics of the spectrum, focusing instead on the general correspondence of the spectra overall.
Numerous patents and commercial applications are found for this basic approach, which is generally called resonant ultrasound spectroscopy (RUS). A group of related approaches teach the use of this method to determine when manufactured components fall outside of some set of specifications. These teach various additions and extensions of the principle, such as using the method to determine sphericity of a given component, temperature compensation for resonant spectra, prediction of resonant frequencies of specific components to allow limiting of the scanned bandwidth in diagnostic testing, and using shifts between “wet” and “dry” spectra to determine the presence or absence of cracks or crack-like flaws in a component. Together, these approaches have resulted in several commercial applications for testing of manufactured components and for determination of characteristics of materials, such as those offered by Magnaflux' Quasar systems and Mechtronic's Vibrant NDT.
All of the above approaches apply to components of a known shape (with some variation depending on dimensional specifications, etc.) placed in a testing location, and isolated from all other components. To date, none of these approaches have been used in field settings, in general because most mounted components have connections/attachments to other components which can suppress, damp, or unpredictably change the expected resonances. The other generally related methods, faced a number of additional difficulties in that they had a bandwidth-limited approach, did not isolate the rail segment, could not use current signal-processing techniques, and so on. Current uses of RUS are for isolated and relatively small components, generally either undergoing post-manufacture inspection or being examined for suspected flaws, or for materials characterization. In both cases, the sample or component is placed in a very specialized holder and isolated. Large components have historically presented issues with the amount of energy needed to properly evoke the resonances.